The present invention relates generally to systems for drying of moisture-containing work pieces, such as lumber or the like, and to methods for drying same, particularly, to ventilation systems and methods for drying the work pieces, and more particularly, to ventilation systems used in conjunction with kilns and methods for drying work pieces such as lumber.
Drying of moisture-containing work pieces, such as lumber, is known. For example, during the kiln drying process of lumber, heat is injected into the internal workspace of the kiln to remove a substantial amount of moisture from the lumber located therein. The moisture removal rate from the lumber is controlled by setting the kiln dry bulb and the corresponding kiln wet bulb temperatures at specified values. These temperatures are determined by the drying schedule selected by the kiln operator. As the lumber dries, moisture is absorbed into the kiln atmosphere which in turn raises the wet bulb temperature.
A wet bulb controller is provided to maintain the wet bulb temperature at a steady level by opening vents located on the kiln. This releases humidified air from the kiln. The humidity is replaced with fresh external atmospheric air that is at a lower temperature and contains less moisture. A substantial portion of the internal atmosphere inside the kiln is transferred out of the exhaust vents, and the fresh air is drawn into the intake vents by the main kiln air recirculation system.
All of the air that is drawn into the kiln has to be reheated to the operating kiln temperature. This is accomplished using the dry bulb controller. The dry bulb controller will act to maintain steady state operating conditions within the kiln workspace by opening a heat control mechanism which injects heat into the kiln as determined by the operator. This injected heat will cause the lumber to release additional moisture. Then, the venting/heating cycle will be repeated as required.
U.S. Pat. No. 5,018,281 discloses a heat exchanger system for a tobacco curing and drying barn 10 that is designed to utilize the heat associated with exhaust air leaving the barn to heat fresh incoming air. A top mounted heat exchanger 50 includes a pair of laterally spaced counterflow heat exchanger modules 52, 54 connected to the barn and to a fresh air duct 74 that leads back into the during and drying structure.
U.S. Pat. No. 5,195,251 discloses a drying kiln including a drying chamber, a heating system for heating air to be delivered to the drying chamber, and a ducting system for delivering heated air to the drying chamber. Fans are provided for establishing a low air pressure region and a high air pressure region adjacent the ducting system for generating a circulation flow of air throughout the drying chamber.
In U.S. Pat. No. 4,662,083, first and second ported ducts are associated with a drying chamber. A flow control housing has first and second compartments communicating respectively with the first and second ducts. These compartments also have openings which communicate with atmosphere. A fan is employed which draws air from the drying chamber into one of the ducts and through one of the compartments to atmosphere. At the same time a fan on a common shaft draws in make-up air from the atmosphere and forces it through the other of the compartments and into the other duct for discharge into the dryer. The fans are reversible, and the flow control housing is combined with a double acting heat exchanger which extracts heat from exhaust air in one compartment and transfers it to the other compartment for preheating make-up air. Since the fans are on a common shaft they cannot act independently but instead are selectively driven in one direction or the other. More specifically, when the fan journal spins in a first direction, the fan 58 acts as an intake fan and the fan 60 acts as an exhaust fan. When the journal direction is reversed, the fan 58 acts as an exhaust fan and the fan 60 acts as an intake fan. This precludes allowing the two fans to operate in either a full exhaust mode or a full intake mode. A damper is provided in the flow control housing on the atmosphere side thereof for precise control of exhaust and make-up air. The fan speed variability and the flow are specifically controlled by adjustment of the damper.